The optical performance of lens machined by single-point diamond ultra-precision turning can be affected by both form errors and surface roughness. The former can be characterized by the spectra of lower spatial frequencies of surface variations while the latter by much higher spatial frequencies. Fast Fourier transform method is used to analyze the surface profile and to decompose the surface features in terms of relative spatial frequencies (or periods). Most of the conventional post-machining lapping processes are aimed at improving the surface finish mainly, and the form accuracy would very likely be tempered. A lapping process with very low lapping pressure is used to study the relationship between the various spatial frequency groups with various lapping process parameters. The preferred spectral group is found to change with lapping time. As the lapping time is increased beyond a certain point, the spatial period of the preferred spectral group shifts to a lower spatial frequency region. Thus, it is possible to improve the surface finish while maintaining the form accuracy. The study would have important implications in the lapping of aspheric or freeform surfaces.